Transportation Deployment Casebook/Interstate Highway System

The Interstate Highway System (officially known as the Dwight D. Eisenhower National System of Interstate and Defense Highways – after the president who signed the authorizing bill) is a network of approximately 46,750 miles of grade-separated, limited-access freeways and highways[1]. It began with the Federal-Aid Highway Act of 1956, which authorized construction of the system, and was intended to connect most major cities and regions in the United States. Today, the Interstate Highway System connects all 48 contiguous states.

Like many transportation and infrastructure systems the Interstate Highway System follows a roughly S-curve shaped pattern of development. Its lifecycle can be divided into three distinct phases of birth, growth, and maturity. In the initial birth of the new transportation technology growth is slow as its role is clarified and norms are established. Once the benefits of the technology begin to be more widely understood, the technology enters a growth phase where expansion occurs rapidly. Investment, public or private, increases and the system gains many new entrants. After the system reaches its maximum extent, it enters the maturity phase of its growth. Typically consolidation occurs, and inefficient or superfluous routes are discontinued.

An analysis of the annual vehicle-miles traveled on the Interstate Highway System shows that it follows this pattern of development. Fitting the S-Curve requires an initial single variable linear regression to determine the b-value coefficient used in the logistic function of the S-Curve. The regression is of the form: Y=bX+c; where Y=LN(Vehicle-miles/(K-Vehicle-miles)), and X=Year K is the maximum extent of the technology, in this case estimated to be 750,000 vehicle-miles Once b is found, it can be used to determine the inflection point (t0=Intercept/-b), and the estimated extent of the technology using the S-Curve. The function is of the form: S(t)=K/[1+exp(-b(t-t0)], where t=time, t0=inflection point, and K=saturation point[4]

Prior to the construction of the Interstate Highway System in 1956, rail was still the most dominant form of long distance transportation for both freight and passenger travel. However the automobile had been replacing rail travel steadily since the middle of the first decade of the 1900s. Rail volume was declining for most of this period, with brief upticks during World War I and World War II due to rationing of car parts, particularly rubber, metals and fuel. However despite these brief setbacks, automobile growth continued to grow rapidly. In 1900, there were approximately 8000 passenger cars in the United States. By 1930 this number had grown to just over 23 million and by 1955 there were 52 million automobiles on the road[5].

The rapid growth of the automobile put considerable stress on the existing roadways in the United States. These roadways were largely one or two lanes, were not grade separated and maintained and funded by states and local municipalities. As such they were unequipped to deal with the growing volume of vehicle use, both in terms of personal automobiles and shipping via trucks. In 1953, the Bureau of Public Roads released a report saying that only 24% of interstate roadway was adequate to meet the demands of current traffic levels[6]. In the 1930s and 40s some regionally contained limited-access divided highways were being constructed and enjoying a large degree of success, notably the Pennsylvania Turnpike which was the first of its kind to be constructed in the United States.

1955 map: The planned status of U.S. highways in 1965, as a result of the developing Interstate Highway System

The success of the early limited-access highways increased pressure on the Federal government to begin the development of a national highway system that could support the growing automobile traffic in the United States. Both President Franklin D. Roosevelt and Congress began exploring the idea of a network of superhighways in the late 1930s and early 40s. Roosevelt, fearing a return of the Great Depression after WWII, wanted to incorporate the construction of the superhighway network into his broader public works investment plans as a means of providing jobs for returning war veterans and other people out of work[7]. In 1938, Congress passed the Federal-Aid Highway Act of 1938, which directed the chief of the Bureau of Public Roads to conduct a feasibility study of six interstate toll road corridors[8]. The study found that in all likelihood, the amount of traffic across these routes would not be sufficient to support a toll road, and that government funding would have to support them in some capacity[9].

Although the idea of an interstate highway system garnered a great deal of support in Congress, and was popular among the public, little progress was made on the matter because the massive investment in the war effort took priority over large infrastructure investments at home. In 1944, the second Federal-Aid Highway Act designated 65,000km of interstate highway system to be selected by the various state highway departments. However no special funding was authorized for the construction of the system, resulting in resistance from states that did not want to reallocate federal aid away from local needs. The first federal authorization of funds specifically for the construction of the interstate highway system came in 1952, with $25 million from the federal government on a 50-50 matching basis[10].

Finally in 1956, a comprehensive program was enacted to begin construction in earnest. President Dwight D Eisenhower, an even stronger advocate for the system than President Roosevelt was, recognized the importance of a national system of interstate highways. In 1919 he was a participant in the Army’s first transcontinental convoy from Washington, DC to San Francisco. The journey suffered a series of mechanical difficulties and infrastructure failures and took over two months to complete. This experience, as well as his experience with the German Autobahn during World War Two had convinced him of the value of the interstate system. In addition to President Eisenhower’s support, the postwar affluence of the United States had increased pressure even further on existing highways and the ramping up of the Cold War provided a national defense justification for the system. In short, by 1956 the Interstate Highway System was an idea whose time had come.

The modern Interstate Highway System began with the Federal-Aid Highway Act of 1956, which was approved by the Senate on June 26th by a vote of 89-1 and signed on June 29th by President Eisenhower[11]. The act authorized 42,500 miles of interstate highway system to be completed by 1975. All interstate highways were to be constructed using very high standards. It was designed to have no intersections and traffic signals, and to be grade-separated at traffic and rail crossings and in urban areas. Grades and curves were designed to reduce or eliminate blind hills and safe navigation at high speeds[12]. The act stated that construction and maintenance was to be funded by a user fee federal gasoline tax. The federal tax was planned to provide 90% of the construction cost, with the remainder coming from the states, mainly in the form of their own user fees. To that end, the federal gasoline tax was increased by one cent.

Following its authorization in 1956, the Interstate Highway System grew rapidly, both in terms of absolute length and in terms of traffic volume. By 1960, more than 10,000 miles had opened, doubling in five years to 20,000 open miles in 1965 and reaching 30,000 miles in 1970[13]. By 1994 the system was approaching its current size in terms of mileage (approximately 42,000 miles), yet it only made up 1.1% of the total mileage extent of transport systems in the United States. Despite its relatively tiny total mileage, it carried approximately 23% of the market share of all transportation systems. The rapid changeover from other modes to the Interstate Highway system reflects its usefulness, particularly between cities. The interstate highways have reduced average travel times between cities by an estimated 20% and some much higher, for example travel time from Atlanta to Birmingham has been reduced by approximately 40%[14].

As previously mentioned, the United States did have some limited-access highways built before the construction of the Interstate Highway System. These parkways and freeways, like the Pennsylvania Turnpike, were largely modeled on the German Autobahn. The interstate system followed suit, integrating the higher design standards, wider shoulders, more steeply curved surface, and separate grades; the interstate system actually incorporates much of the Pennsylvania Turnpike, as well as some other early limited-access highways.

In order to successfully complete the system, highway engineers needed to implement newly advanced technologies and improve on many existing ones. Asphalt had improved greatly in World War II, largely because of the requirements of military aircraft runways. Highway engineers developed larger equipment, such as electronic leveling controls and extra-wide lane finishers to pave two lanes at a time[15]. Other major advancements in road technology include drainage, the movement from clay culverts to metal and concrete, and bridge spanning technologies like pre-stressed concrete and segmental construction.

The impact the Interstate Highway System has had on the social, economic, and cultural life in the United States in undeniable. By significantly reducing travel times, especially across large distances, it has helped the truck shipping industry develop into its modern form. Freight costs have been reduced substantially, with tractor-trailer operating costs estimated to be 17% lower on interstates than on other highways[16]. Improved freight shipping translates into inventory benefits for retailers who practice “just-in-time” delivery, and also reduces costs passed on to consumers in the form of shipping.

Reductions in travel costs as a result of the Interstate System have also had an impact on land-use and development in the United States. The development of the interstate system coincides almost directly with the growth of “suburban sprawl”. While there are likely many factors at work behind increased suburbanization, data suggests that the interstate highways are at least partially responsible. There are some estimates that indicate that if the Interstate Highway System had never been built intra-city commutes would double, while suburb to suburb commutes would be halved[17].

Through 1996, close to the final extent of mileage construction, the Interstate Highway System had cost $329 billion dollars (1996 dollars), making it one of the largest infrastructure investments in human history, and the largest in U.S. history. Its original cost was estimated to be $41 billion in 1957 dollars, fairly close to its total cost of $58.5 billion adjusted to 1957 dollars[18].

Interstate construction and maintenance is funded primarily through user fees in the form of gasoline taxes. The federal government’s contribution is around 93% supported by the gas tax. State and local contributions vary, but generally come from property taxes or some means other than a user fee[19]. There is very little money allocated to the expansion of the Interstate Highway System for at least two reasons. The first is that the system has reached its planned extent, and while there are some other expansions slated, the bulk of the work has been done. The second is the increased maintenance costs associated with the expansion of the suburbs. As development springs up along the interstate highways, maintenance costs of those stretches increase through higher use, increased access points, and more grade-separated crossings.

While the Interstate Highway System has reached maturity, it is not going to be replaced any time soon. There is currently no transportation technology that has the capacity to provide the level of service, at the level of cost, that the Interstate Highway System does. There may be some upgrades in electronic systems or “smart roads” to take advantage of the possible computerization of personal automobiles and freight trucks. Depending on how that technology develops, the Interstate System could experience a second life-cycle process that capitalizes on the significant increases in capacity, speed and overall efficiency a fully computerized highway system could provide.